This invention relates generally to electrical interconnections for connecting printed circuit boards.
Electrical connectors are used in many electronic systems. It is generally easier and more cost effective to manufacture a system on several printed circuit boards (“PCBs”) that are connected to one another by electrical connectors than to manufacture a system as a single assembly. A traditional arrangement for interconnecting several PCBs is to have one PCB serve as a backplane. Other PCBs, which are called daughter boards or daughter cards, are then connected through the backplane by electrical connectors.
Electronic systems have generally become smaller, faster and functionally more complex. These changes mean that the number of circuits in a given area of an electronic system, along with the frequencies at which the circuits operate, have increased significantly in recent years. Current systems pass more data between printed circuit boards and require electrical connectors that are electrically capable of handling more data at higher speeds than connectors of even a few years ago.
In some instances, differential connectors are used to carry high speed data. Examples of differential electrical connectors are shown in U.S. Pat. No. 6,293,827, U.S. Pat. No. 6,503,103, U.S. Pat. No. 6,776,659, and U.S. Pat. No. 7,163,421, all of which are assigned to the assignee of the present application and are hereby incorporated by reference in their entireties.
Printed circuit board design also contributes to the ability of an interconnection system to carry multiple high speed signals. Printed circuit boards are manufactured with layers of conductive traces that form microstrip transmission lines. Each microstrip transmission line generally has a uniform impedance, allowing even a high frequency signal to propagate along it with relatively low loss or distortion.
To make connections between microstrip transmission lines on different layers or between microstrip transmission lines within the printed circuit board and components on the surface, of the printed circuit boards, vias are formed. Vias are formed by drilling a hole fully or partially through the printed circuit board and plating the hole with a conductive material. Unfortunately, the via creates a disruption in the uniform impedance of the microstrip transmission line and therefore diminishes the integrity with which signals are propagated. Consequently, techniques have been developed to reduce the size of vias or to eliminate the need for vias altogether. For example, it is known to route traces associated with signals that are to be connected together on the same layer of a printed circuit board so that they can be interconnected without vias. It is also known to route traces carrying high frequency signals on the surface of the printed circuit board to enable connections to be made to the traces without the need for vias. It is also known to decrease the size of vias, such as by backdrilling, which involves drilling away portions of the via after it is formed, or by using buried vias or blind vias, which involve forming vias only a portion of the way through a printed circuit board.
It is also known to improve signal integrity of electronic systems through the use of capacitors attached to signal lines. Capacitors block the flow of low frequency signals, which, in a high frequency system, may be noise. Capacitors may also be connected adjacent inductive segments along a signal conductor to counteract the effect of the inductive segment on the impedance of the signal conductor.